(1) Field of the Invention
The present invention relates generally to a power management method for a modular battery.
(2) Description of the Prior Art
High power level rechargeable batteries are often necessary for specific applications. These batteries are made up of a plurality of series connected cells grouped in modules. In electric vehicle applications, batteries such as this are often capable of delivering in excess of 100 KW at a voltage of 400 VDC or above. A battery of this nature can have in excess of one hundred cells divided into individual modules or banks of cells.
A battery of this power level must be handled very carefully and monitored very closely during charge and discharge. This is particularly true of lithium ion batteries. The voltage of each cell must be monitored during discharge to ensure that no cell voltage is allowed to drop below approximately 2.1 VDC. Allowing a cell voltage below this level may cause irreversible damage to the cell. More importantly the voltage of each cell must be carefully monitored and controlled while charging overcharging a cell beyond approximately 4.3 VDC can result in catastrophic failure of the cell.
It is also desirable that all cells remain at the same voltage level, charge at the same rate, and reach the desired final voltage level at the same time. In practice, however, this is not the case; cell charge and discharge characteristics vary. All cells must be monitored very closely during charging. If any cell (or group of cells) reaches a predefined upper voltage limit prior to the rest of the cells, charging is suspended and the rogue cells must be discharged to the voltage level of the other cells. This process is referred to as “cell equalizing” or “cell equalization”.
Monitoring battery temperature during charge and discharge is also very important. Cell temperatures should not be allowed to exceed a predetermined temperature. If this temperature is reached, corrective action must be taken immediately. This corrective action can include shutting down the charge or discharge process or activating a cooling system.
These problems are specifically acute when using lithium-ion batteries; however, other battery chemistries have similar problems. In view of the prior art, there is a need for a battery monitoring and charging system that allows full monitoring and control of the battery.